1. Field of the Invention
The present invention relates to an optical waveguide and a thermal assist magnetic recording head (or thermally-assisted magnetic recording head) that uses the same.
2. Description of Related Art
Further improvement to thin film magnetic heads and magnetic recording media is in demand in conjunction with the promotion of high recording density in magnetic disk devices in the field of magnetic recording using a head and medium. At the present time, composite type thin film magnetic heads, including a structure where a magnetoresistive (MR) element for reading and an electromagnetic conversion element for writing are laminated, are widely used as thin film magnetic heads.
Meanwhile, the magnetic recording medium is a so-called discontinuous body on which magnetic microparticles aggregate, and each of the magnetic microparticles is a single magnetic domain structure. Here, a single recording bit is configured from a plurality of magnetic microparticles. Accordingly, in order to increase the recording density, unevenness of the recording bit boundary must be reduced by reducing the size of the magnetic microparticles. However, reducing the size of the magnetic microparticles produces the problem of a reduction in the thermal stability of magnetization that accompanies volume reduction.
Increasing the magnetic anisotropy energy Ku of the magnetic microparticles can be considered as a countermeasure for such a problem. However, an increase in Ku causes an increase in the anisotropic magnetic field (coercive force) of the magnetic recording medium. In this regard, the upper limit of the write magnetic field intensity by the thin film magnetic head is nearly determined by the saturation magnetic flux density of the soft magnetic material that constitutes the magnetic core within the head. Accordingly, writing becomes impossible when the anisotropic magnetic field of the magnetic recording medium exceeds the tolerance determined from the upper limit of the write magnetic field intensity. Currently, one method to resolve the problem of this type of thermal stability proposes a so-called thermally-assisted magnetic recording method in which writing is performed by reducing the anisotropic magnetic field through adding heat to the magnetic recording medium immediately prior to applying the write magnetic field while using a magnetic material with a large Ku.
A commonly known method for such thermally-assisted magnetic recording uses a near-field probe, a so-called plasmon antenna, that is a piece of metal that generates a near-field from plasmon excited by an irradiated laser beam. For example, a plasmon-generator is disclosed in the specification of U.S. Pat. No. 6,768,556 that provides a cone shaped metal scatterer formed on a substrate, and a film, which is dielectric or the like, formed around the periphery of the scatterer.
Further, a configuration is disclosed in U.S. Patent Publication No. 2004/081031 A1 that forms a plasmon-generator in a position to contact the main magnetic pole of a perpendicular magnetic recording head so that the irradiated surface is perpendicular to the magnetic recording medium. Furthermore, technology is disclosed in U.S. Patent Publication No. 2003/066944 A1 that attempts irradiation of a stronger near-field onto the magnetic recording medium by making a priority for the tip of the plasmon antenna to be closer to the magnetic recording medium.
The inventors of the present application are proceeding with the development of a further improved thermally-assisted magnetic recording head by using near-field irradiation in pursuit of the limit of magnetic recording potential.
When performing thermally-assisted recording with a magnetic recording head using irradiation of a near-field, a laser generating device is mounted on the magnetic recording head as a light emitting element, and the laser light emitted from the laser generating device is introduced into an optical waveguide and requires guidance to the plasmon antenna which exists in a position in close proximity facing the magnetic recording medium.
A spot size converter is used at such time to effectively taper the spot size of the laser light entering the optical waveguide. However, the size of the magnetic recording head itself is extremely small, so that a design that reduces the size of the spot size converter by shortening the waveguide length to effectively taper light to be suitable for use in such a corresponding size is desired to provide a spot size converter with favorable spot size conversion efficiency even with a reduced size. Further, with a use of a thin film technology, an optical waveguide having a spot size converter that is formed with a high productivity, even though its size remains minute (or fine), is desired. For example, it is desired to have a spot size converter in which its minute pattern forming film rarely breaks.